The invention concerns a material for passive electronic components with high thermal conductivity, low density and low coefficient of thermal expansion.
The term "passive electronic components" here refers to those not directly involved in electronic activity. The invention relates more particularly to materials for making structures such as sinks, supports and pole pieces in power circuits, laser diode supports, heat sinks and encapsulating cases in hybrid microelectronic power circuits and ultra high frequency circuits. The term electronic also includes the optoelectronic field.
In the components in question these materials are known to be joined generally to insulating ceramic substances such as alumina or semiconductors such as silicon or gallium arsenide.
If the components include power elements, a large amount of heat is emitted when they operate. This has to be dissipated as rapidly as possible, to avoid damaging the components by excessive heating. A material of the highest possible thermal conductivity is therefore used.
The temperature nevertheless rises, and if the coefficient of thermal expansion of the material differs too much from that of the ceramic insulator or semi conductor substrate, the stresses which are set up in the substrate are greater than the resistance of the ceramic. The ceramic therefore breaks, impairing the efficiency of the whole unit. Thus it is also necessary for the material to have a coefficient which is compatible with that of alumina or silicon and preferably below 16.times.10.sup.-6 .multidot.K.sup.-1 in the 30.degree.-400.degree. C. temperature range.
The fact that these circuits may be used in vehicles driven by a source of energy has led to a search for materials with the lowest possible density, preferably below 3100 kg.m..sup.-3, in order to minimise the energy consumption required to propel the vehicles.
Since the circuits are affected by their surroundings, the material should also have a suitable amagnetic character and good resistance to the external environment.
A large amount of research has been carried out to find a material which would form a compromise between all these properties, and there have been more or less interesting findings.
Thus materials such as steel, beryllium and some aluminium alloys have been tried because of their good conductivity; but their relatively high modulus of elasticity and expansion capacity make it necessary to use joints or adhesives to accommodate the difference from the expansion capacity of the alumina, and this reduces the thermal conductivity of the whole unit.
Researchers then turned to materials with low expansion capacity, such as kovar (iron-nickel-cobalt alloy) or molybdenum or multi metal materials of the copper/invar/copper type as well as titanium and its alloys. But apart from molybdenum all these materials are handicapped by low conductivity, particularly in the direction perpendicular to the plane of the substrate. They all have high density also; the lowest density, that of titanium, is of the order of 4500 kg.m..sup.-3. In addition molybdenum is expensive and difficult to use because of its poor resistance to oxidation; as for kovar, it is tricky to machine as it is twisted by internal stresses, and many annealing operations are necessary if it is to be worked correctly.